1. Technical Field
The present invention relates to the lithographic writing of patterns on substrates by means of directed energy beams, typically electron beams. More particularly, the present invention relates to increasing the throughput of beam lithography by utilizing bi-directional scanning of the beam over the substrate including correction of the beam scanning characteristics in both scan directions to achieve high accuracy in the patterns being written.
2. Description of Related Art
The fabrication of integrated circuits (xe2x80x9cICsxe2x80x9d) requires ever more accurate methods for creating patterns on a wafer substrate. Two basic processes are commonly used. In one process, the pattern may be created on a resist-coated wafer by exposing the wafer to a beam of energy directed onto the substrate through a mask containing the desired pattern. xe2x80x9cPositive resistsxe2x80x9d require transparent regions of the mask for those areas in which resist removal is desired. xe2x80x9cNegative resistsxe2x80x9d require opaque regions of the mask for those areas in which resist removal is desired. Both positive and negative resists are commercially useful. Exposure through a mask is typically performed with electromagnetic radiation although exposure by means of electron, ion or particle beams impinging on the mask are not excluded. xe2x80x9cPhotolithographyxe2x80x9d denotes the exposure of a resist-coated substrate through a patterned mask, typically by means of electromagnetic radiation.
The second method of writing patterns on a resist makes use of a beam of energy directed only to those regions of the resist-coated surface requiring exposure without screening by an intervening mask. A suitable beam steering mechanism is typically employed along with suitable on-off controls to insure that only the regions of the surface requiring exposure are contacted by the incident beam. The beam may be electrons, ions, neutral particles or collimated laser light or other electromagnetic radiation. However, to be definite in our discussion, we will emphasize the example of a beam of electrons impacting the resist-coated substrate (e-beam lithography), not excluding thereby other forms lithography by means of directed energy beams.
Direct beam writing of patterns onto a resist-coated surface is the method presently preferred for creating the masks used in photolithography, but the technique offers other advantages as well. Among these other advantages of direct beam writing are the avoidance of complications of alignment and registration of the mask with the substrate and the possibility of creating more precise patterns with the use of accurately focused beams. One disadvantage of direct beam patterning in comparison with photolithography is the relatively smaller throughput possible with direct beam writing. Increasing the throughput of direct beam writing is one objective of the present invention.
Considering e-beam lithography by way of example and not limitation, the presently employed writing techniques may be classified into one of two general categories, vector scan or raster scan. Vector scan typically directs the beam while off to a region of the substrate requiring exposure, then by turning the beam on exposes a contiguous region of the substrate to the energy of the beam before moving to another region for exposure. Simply stated, vector scanning xe2x80x9cpaintsxe2x80x9d or xe2x80x9ctilesxe2x80x9d a region of the substrate with beam energy before moving on to expose another region. Most conveniently, beam direction, scanning trajectories, pixel spot size and/or intensity are under computer control, defining the pattern to be written.
Raster scanning directs the beam to all regions of the substrate no matter what pattern requires exposure and adjusts the beam intensity at each point scanned to effect the correct pattern of exposure. The simplest beam control during raster scanning entails having the beam on or off as each pixel is scanned. However, adjustment of beam intensity to numerous levels between full-on and full-off (gray scales) is also feasible in some raster scanning procedures. For example, see the work of Abboud et. al. U.S. Pat. No. 5,393,987.
One common type of raster scanning is unidirectional, denoting the scanning geometry of writing an entire line from beginning to end followed by beam-off xe2x80x9cflybackxe2x80x9d to begin writing the next line adjacent to the first pixel of the just-completed line, as depicted schematically in FIG. 1-I. Bi-directional scanning denotes the writing of a line in one direction (say bottom to top) then writing the immediately adjacent line in the opposite direction (top to bottom) in which the first pixel of the second line is adjacent to the last pixel of the first line, as depicted schematically in FIG. 1-II. The non-productive xe2x80x9cflybackxe2x80x9d period is typically much shorter when bi-directional scanning is used than when writing by means of the uni-directional scanning of Figure 1-I.
Usage in the art may use xe2x80x9craster scanxe2x80x9d as a generic term to distinguish xe2x80x9cvector scanxe2x80x9d including within the concept of raster scanning both uni-directional and bi-directional scanning. Unfortunately, xe2x80x9craster scanxe2x80x9d may also be used to indicate just uni-directional scanning as in FIG. 1-I in contrast to xe2x80x9cserpentine scanningxe2x80x9d of FIG. 1-II. Hereinafter, we will use xe2x80x9craster scanxe2x80x9d to distinguish vector scan to indicate that every pixel in the scan pattern has the beam directed in its direction with the beam off for pixels not written, including therein both unidirectional and bi-directional scanning of FIG. 1. To be definite in expression, FIG. 1-I we denote as xe2x80x9cunidirectionalxe2x80x9d while FIG. 1-II we denote as either xe2x80x9cbi-directionalxe2x80x9d or xe2x80x9cserpentine.xe2x80x9d
xe2x80x9cWriting a linexe2x80x9d is used herein to distinguish from xe2x80x9cflyback,xe2x80x9d even though few or none of the pixels in a particular line xe2x80x9cwrittenxe2x80x9d actually receive any beam energy. That is, xe2x80x9cwriting a linexe2x80x9d denotes scanning the beam over a line of pixels on the substrate under a condition that, depending on the particular pattern, may or may not receive beam energy striking pixels in the line. In contrast, xe2x80x9cflybackxe2x80x9d denotes the act of repositioning the beam following completion of the writing of a particular line in preparation for writing the next line of the pattern. During flyback, the beam is off. That is, the beam is typically directed to a blanking plate, beam dump, or in some other fashion caused not to impact onto the substrate although an actual energy beam may be generated in the writing system. Genuine termination of the beam at its source is also possible for the xe2x80x9cbeam-ofxe2x80x9d condition. Thus, the beam is typically fully off during flyback but, during writing, impacts each pixel in the line or pattern being written with the appropriate amount of energy (which may be zero).
Writing precise patterns on the substrate requires (among other things) precise control of the beam path during writing and precise positioning of the beam during flyback to insure correct placement of lines on the substrate. Beam deflection signals typically include both electronic and electron optic effects to direct the beam in the desired writing-pattern. However, such beam deflection signals are typically not linear to the required degree of accuracy, leading to imprecise patterns. One way to achieve high accuracy in e-beam writing and positioning is to measure the beam distortion in a calibration step and apply beam correction signals (xe2x80x9cdynamic correctionsxe2x80x9d) during actual writing. Prior work of the present inventor related to uni-directional scanning, U.S. Pat. No. 5,345,085 (xe2x80x9c""085xe2x80x9d) describes methods and systems for determining the dynamic corrections during a calibration step, storing correction signals in a table look-up arrangement then applying the proper correction signals to the main beam deflection signal as the pattern is being written. Increased pattern accuracy results.
However, since the flyback time for unidirectional scanning is typically larger than the flyback time for bi-directional scanning, throughput of e-beam patterning systems may be increased if bi-directional scanning is employed. The work of Yew (U.S. Pat. No. 4,445,039) is one approach to bi-directional scanning in which savings in flyback time translates into increased system throughput.
Several challenges must be met in generalizing the beam correction techniques of the ""085 patent to bi-directional scanning, particularly when beam accuracy around one part-per-million (1 ppm) is required. For example, the scan distortions for the positive scan direction (upward, line 2c, in FIG. 1-II) are typically significantly different from those in the negative scan direction (line 2d in FIG. 1-II). This is illustrated in FIG. 1A in which a dynamic correction signal, 200c for scan 2c is qualitatively and quantitatively different from the dynamic correction, 200d, that needs to be applied to the opposite-running scan line, 2d. The depictions of FIG. 1A are schematic and illustrative only, as the dynamic correction signals (200c, 200d) typically represent voltages applied to the beam deflection means to correct for inherent imperfections in the beam deflection circuitry, while the scan lines are depicted in a two-dimensional spatial plane. Since the dynamic correction signal correlates with the location of the beam along its scan line, it is convenient to depict both on a single figure as in FIG. 1A, notwithstanding the combining of distance and voltage units of measure.
This asymmetry in scanning properties seems to derive chiefly from electronic effects, such as the positive and negative directional scanning making use of drive circuits that are not precisely symmetrical in reversing directions. At a minimum, the table of corrections doubles in size as independent corrections for reversed scan directions need to be included.
Other effects leading to beam distortions include eddy current or skin effect distortions of the signals applied to deflection coil(s), phase delay asymmetries in positive and negative scans, position shift in the last pixel of one scan line and the first pixel of the immediately following scan line, and others. Each distortion typically has its own cause and each must be categorized independently to insure that the proper correction is applied to the deflection signal in both positive and negative scan directions.
Additional complexities are introduced into the system timing when bi-directional scanning is employed. One objective in writing patterns is to properly align the pixels of one scan line with the corresponding pixels of the immediately adjacent scan line. Typically, accuracy better than 5 ppm is desirable in this line-to-line alignment of individual pixels. The timing of individual system signals may have a substantial effect on the positioning accuracy of individual pixels, including (but not limited to) timing and control of the deflection signal, beam blanking control (beam-on, -off), dynamic correction signals and error correction signals compensating for mechanical motions of the substrate being written (typically a mechanical stage holding the substrate to be patterned).
A portion of a bi-directional scan is depicted in FIG. 2 in which 7 denotes a sequence of individual pixels, solid circles indicating written pixels and open circles indicating unwritten pixels. We depict writing as either xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d merely for convenience and not to indicate that the present invention excludes intermediate levels of pixel exposure between full on and full off. FIG. 2A depicts two lines of writing side-by-side and both starting and ending pixels. The left depiction, 2A(I) indicates perfect alignment while the right depiction, FIG. 2A(II) indicates possible pixel shift from the last position of one scan line to the first pixel of the next scan line. That is, row i, pixel N misaligns with row (i+1), pixel 1. This is a challenge that must be met in an accurate bi-directional scanning apparatus.
Also, the finite bandwidth of typical beam deflection circuits can introduce substantial errors in phase delay timing as depicted in FIG. 2B. The bandwidth, fc, of a typical low pass filter causes a phase delay in the output signal. The delay in FIG. 2B is [1/(2xcfx80fc)] for a triangular input signal. The offset error depicted in FIG. 2B is the product of the delay and the signal slope. Typical values for fc may be approximately 500 KHz. Typical signal slope may be approximately (1000 xcexcm )/(25 xcexcsec), yielding an offset error of 12.7 xcexcm. This offset error, depicted in FIG. 2A, has a detrimental effect on pixel alignment.
A typical application for e-beam patterning is in the creation of photolithography masks for the fabrication of integrated circuits. Such masks typically require numerous identical patterns to be produced at various locations on the mask and interconnected in various ways. It is common for the mask designer to write computer commands for mask creation hierarchically; that is, constructing the desired patterns out of sub-patterns, sub-sub-patterns and the like down to the level of the most elementary pattern written on the substrate by the beam. This hierarchy of function calls must be translated into pixel-by-pixel beam control instructions at the time of writing. Translational of the hierarchical instruction file into the pixel-by-pixel xe2x80x9cflat filexe2x80x9d is typically done as needed during beam writing as the flat file typically contains excessive data (perhaps 1012 to 1015 bytes) that cannot feasibly be created and stored prior to patterning. Thus, the need for real-time data preparation is a complicating factor in generalizing accurate beam writing technologies to bi-directional scanning.
In many beam lithography applications, precise control of both the frequency and amplitude of beam deflection/scanning signals is required. For complex beam lithography systems, timing must be synchronized between the various modules that manage the scanning operations and data preparation. Bi-directional scanning introduces serious complications into many of these steps.
The present invention relates to a bi-directional lithography system, including the generation and application of beam correction signals to achieve higher throughput without sacrificing patterning accuracy.
The present invention relates to bi-directional raster scanning (xe2x80x9cserpentine scanningxe2x80x9d) for an electron beam lithography system. Bi-directional scanning requires true triangular waveforms, typically more difficult to generate accurately than sawtooth waveforms typical of unidirectional scanning. Improved circuit components are described as well as improved data preparation and handling for optimized control of bi-directional scanning.
The circuitry of the present invention provides control of signal amplitude, frequency and phase to better than 1 ppm by making use of signal generation circuitry described in detail herein. A feature of the signal generation circuitry is to provide accurate synchronization of deflection to provide accurate control of pixel location. Improper selection and routing of timing signals can lead to harmful levels of noise and unacceptable signal jitter, the avoidance of which is an important objective of the present invention. A series of coarse and fine tuning delay adjustments pursuant to the present invention accommodate a variety of operating conditions.
Another class of improvements in connection with the present invention relates to an optimization of the data path hardware for bi-directional scanning, including isolating a section of the data path hardware specifically to provide calibration scans. The sawtooth waveform for unidirectional scanning is replaced with a triangular waveform for bi-directional scanning by means of, in one embodiment, using a first-in-first-out (xe2x80x9cFIFOxe2x80x9d) register in the data preparation hardware. Such hardware is provided to provide the data direction control determining the direction of the particular scan line as well as phase control or data delay circuit for shifting the entire scan line by up to several hundred pixels). The delay circuit built into the data preparation hardware helps compensate for delays caused by finite bandwidth in the analogue drivers.
In addition, a two-level beam deflection is described in which fast, low-voltage electrostatic deflectors are employed (preferably one for each scan direction) in conjunction with a slower magnetic beam deflector. The two level deflection system pursuant to the present invention allows for controlling the retroscan signal to match the polarity of the main magnetic deflection signal.